| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 18°F | 34°F | 25°F | 3.2" |
| February | 19°F | 36°F | 27°F | 3.2" |
| March | 32°F | 51°F | 41°F | 5.1" |
| April | 42°F | 64°F | 52°F | 6.5" |
| May | 53°F | 74°F | 63°F | 7.0" |
| June | 62°F | 82°F | 72°F | 7.0" |
| July | 65°F | 85°F | 74°F | 7.2" |
| August | 64°F | 84°F | 73°F | 8.3" |
| September | 57°F | 78°F | 66°F | 7.1" |
| October | 45°F | 65°F | 54°F | 6.0" |
| November | 34°F | 52°F | 42°F | 4.6" |
| December | 23°F | 38°F | 30°F | 3.8" |
| Index | Chillicothe | Illinois | National |
| Air quality index | 41 | 41 | 42 |
| Pollution index | 339,156 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 285 |
| Days with moderate air quality | 78 |
| Days w/ poor A.Q. for sensitive groups | 2 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.000% |
| Benzene | 0.020% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.000% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| La Rose, IL | 114 | 41 | 18.0% |
| Varna, IL | 456 | 41 | 18.0% |
| Peoria Heights, IL | 5,825 | 41 | 18.0% |
| Eureka, IL | 5,485 | 41 | 18.0% |
| Dunlap, IL | 1,100 | 41 | 18.0% |
| Chillicothe, IL | 6,326 | 41 | 18.0% |
| City | Population | Air Quality Index | National Avg. |
| Carolina Beach, NC | 6,324 | 34 | 32.0% |
| West Point, NY | 6,328 | 36 | 28.0% |
| Perth, ON | 6,328 | n/a | |
| Henderson, TN | 6,323 | 27 | 46.0% |
| Castroville, CA | 6,332 | 37 | 26.0% |
| Chillicothe, IL | 6,326 | 41 | 18.0% |
Here are the best places to live in Illinois
Check out the 10 safest cities in Illinois
Check out the 10 most dangerous cities in Illinois
Most Dangerous Cities In The US